1. Field of the Invention
The present invention relates to an electroluminescent device (hereinafter, an EL element) having a film comprising an organic compound (hereinafter, an electroluminescent film) that emits light by applying an electronic field, an anode, and a cathode. More particularly, the present invention relates to an EL element that has good yields, high reliability, and low-voltage drive; and a light emitting device that includes the EL element. A light emitting device in this specification includes an image display device including an EL element, and also module in which a connector, a flexible printed circuit board (FPC), TAB (tape automated bonding), or TAB (tape carrier package) are attached to an EL element, or module in which a print circuit plate is attached to a tip of the TAB or TCP, or module in which IC (integrated circuit) is mounted directly on an EL element by COG (Chip On Glass).
2. Description of the Related Art
The EL element includes an organic compound as a ruminant object emits light by being applied electric field and current. The luminescence mechanism is described as flowing: by applying a voltage to an electroluminescent film sandwiched between electrodes, electrons injected from the cathode and holes injected from the anode are recombined in the electroluminescent film to form molecules in an exciting state (hereinafter referred to as “molecular exciton”); and energy is radiated when the molecular exciton moves back toward the ground state thereof.
The kind of the molecular exciton which is made from the organic compound may be a singlet exciton state or a triplet exciton state. In the present specification, luminescence may be based on any one of the two.
In such an EL element, an electroluminescent film is generally formed into a thin film to have a thickness of approximately 100 nm. In the EL element, the electroluminescent film emits light spontaneously, therefore, backlight used in the conventional liquid crystal displays is unnecessary. As a result, the EL element has a great advantage of being fabricated thinly and lightly.
Also, for example, a period from the injection of a carrier to the recombination of an electroluminescent film having a thickness of approximately 100 nm is about several ten nanoseconds considering the carrier mobility. Even when a process from the recombination of a carrier to light emission is included in the period, light is emitted within the order of microsecond. Thus, an extremely high response speed is one of characteristics thereof.
Further, since the EL element includes an organic compound as a ruminant object is a carrier injection type device, it can be driven by a direct current voltage, thereby noise is hard to generate. With respect to a drive voltage, an electroluminescent film is fabricated into a uniform ultra thin film having a thickness of approximately 100 nm, an electrode material such that a carrier injection barrier to the electroluminescent film is decreased is selected, and a hetero structure (two-layers structure) is introduced. Thus, a sufficient luminance of 100 cd/m2 at 5.5V has been achieved (Reference 1: C. W. Tang and S. A. VanSlyke, Applied Physics Letters, vol. 51, No. 12, pp. 913–915 (1987)).
The EL element includes an organic compound as a luminant object has characteristics such as a thin type, lightweight, high speed responsibility, and direct-current low-voltage drive. Owing to this, the EL element is noted as a light emitting device for a next generation flat panel display. In addition, since the EL element is a self-luminous type and has a wide viewing angle, the visibility is relatively good. Thus, it is considered that the EL element is preferable for a display screen of a car mounted electronic appliance or mobile appliance. In fact, the EL element is used for area color of display screen of mounted type car audio.
The EL element disclosed in Reference 1 is adapted so-called “division of function” wherein a hole transporting layer transports a hole and an electron transporting light emitting layer transports of an electron and emits light. The conception of the division of function is developed to a conception of a double hetero structure (three-layer structure) wherein a light emitting layer is sandwiched between a hole transporting layer and an electron transporting layer (Reference 2: C. Adachi, S. Tokito, T. Tsuitsui and S. Saito, Japanese Journal of Applied Physics, vol. 27, No. 2, L. 269–271 (1988)).
One of advantages of the division of function is that there is no need for giving various functions (light emission, carrier transportation, carrier injection from an electrode, or the like) to one kind of organic material, in consequence, molecular design freedom etc. can be expanded widely (for example, it is not necessary to find bipolar materials). By combining a material having good light emitting characteristics, high transportability, or the like, high luminous efficiency can easily achieved.
With respect to division of function, a conception of an anode buffer layer or a cathode buffer layer is suggested for injecting carriers to reduce drive voltage. For example, there is a report that the drive voltage can be reduced by enhancing injection of carrier by means of inserting materials that ease energy barrier into an interface between cathode and the electroluminescent film (Reference 3: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama, Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. 44, No. 8, 1245–1248 (1997)). In Reference 3, it is disclosed that Wakimoto et al. succeeded to reduce drive voltage by using Li2O as a cathode buffer layer.
With respect to a buffer layer, a buffer layer comprising polymer (hereinafter, a polymer buffer layer) attracts especially attention in recent years (Reference 4: Yoshiharu Sato, Molecular Electronics and Bioelectronics (The Japan Society of Applied Physics), vol. 11, No. 1, 86–99 (2000)). In Reference 4, it is disclosed that using a polymer anode buffer layer promotes the lower voltages, longer lifetime, and higher heat resistance. It can be said that it has an effect on preventing short-circuiting.
Since the conductivity of a polymer buffer layer is improved by introducing appropriate acceptor or donor thereinto, the EL element including the polymer buffer layer can be formed into thick that could not emit light unless it has a thickness of approximately 100 nm, normally. Consequently, it is disclosed that the electroluminescent film can be flattened releasing adverse effects of contaminants or projections on an electrode in Reference 4. Thus, there is a risk for short-circuiting due to contaminants or projections on an electrode conventionally, however, the polymer buffer layer is effective in reducing the risk. According to this, there is also advantage that the improvement in yields can be expected.
Therefore, above-mentioned polymer buffer layer can solve simultaneously the conflictive two objects of thickening the film and reducing the drive voltage that has been a serious dilemma for an EL element.
In order to use a polymer buffer layer for thickening the film, water is often used for solvent. However, water solvent is not preferable for EL element having an organic compound because the organic compound is easily damaged by moisture. According to Reference 4, some organic solvent can be used, however, generation of impurities cannot inevitable as far as wet process is used. It may cause the deterioration of the EL element.
When the EL element including the buffer layer made from these materials is used for respective pixels in matrix of display device, a problem of crosstalk is arisen. In more detail, leakage current is caused in places between the polymer buffer layer and wirings (or between pixels) because almost polymer buffer layer is gave conductivity and is generally spin coated on whole surface.
For example, it is reported that crosstalk is caused by fabricating a passive matrix display device that has an anode buffer layer made from polyethylene dioxythiophene/polystyrene sulfonate (hereinafter referred to as “PEDOT/PSS”) that is conductive polymer added with acceptor (Reference 5: A. Elschner, F. Jonas, S. Kirchmeyer, K. Wussow, Asia Display/IDW '01, 1427–1430 (2001)). In Reference 5, it is reported that crosstalk is prevented by increasing the resistivity of PEDOT/PSS on purpose. However, if the resistivity is increased, the polymer buffer layer cannot formed to be thick (that is, current become difficult to be applied to the EL element). Therefore, an advantage of preventing short-circuiting by flattening an electrode surface as a result of thickening of the electroluminescent film will be lost. In addition, an advantage of low drive voltage will also lost because increasing the resistivity causes high drive voltage.
On the other hand, means for fabricating a thick EL element from an organic compound and acceptor (or donor) by co-evaporation in vacuo consistently (Reference 6: J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo, Applied Physics Letters, vol. 73, No. 6, pp. 729–731 (1998)). In Reference 6, it is disclosed that a thick EL element that is driven at low drive voltage can be fabricated from VoPc as a hole transporting material and F4-TCNQ as an acceptor by co-evaporation.
By this method, generation of impurities can be reduced, moreover, a problem of crosstalk can be prevented because patterning can be conducted using shadow masks. However, there is a problem, which will influence lifetime of the EL element, of chemical stability of acceptor or donor that will be doped. The problem of chemical stability of acceptor or donor will be arisen in the case of using a polymer buffer layer. A conception that is differ substantially from above is reported that a novel EL element structure wherein thickening the film causes increasing current efficiency (Reference 7: J. Kido, J. Endo, T. Nakada, K. Mori, A. Yokoi, T. Matsumoto, 49th Japan Society of Applied Physics and Related Societies, p. 1308, 27p-YL-3, (March 2002)). Conducting means disclosed in Reference 7 brings an advantage that the current efficiency can be improved, however, more deal of voltage is needed to apply compared to the conventional EL element.